The present invention relates to a pneumatic system for controlling the application of liquid adhesive to sheets driven past an applicator and, more specifically, to such a system for an adhesive binder.
Commonly assigned U.S. Pat. No. 4,406,247, issued Sept. 27, 1983 in the names of R. C. Baughman et al and entitled "Adhesive Dispensing System" discloses an adhesive applicator having a nozzle through which liquid adhesive is dispensed onto sheets as the sheets are driven seriatim past the nozzle. The sheets are assembled one above another and pressed together in the area of the adhesive in order to form bound booklets. Sheets driven toward the applicator are detected by a light emitter and optical detector, and the resulting signal is provided to a logic and control unit of the binder. The logic and control unit, in turn, activates a solenoid controlled valve in the applicator to turn the applicator on just before the leading edge of a sheet reaches the applicator nozzle and to turn the applicator off just after the trailing edge of the sheet passes the nozzle. In this manner, a continuous stripe of adhesive is applied to one surface of the sheet.
While the apparatus disclosed in U.S. Pat. No. 4,406,247 works satisfactory for its intended purpose, there are several disadvantages to the apparatus. First of all, the time required for it to operate may be longer than desired under certain circumstances. More specifically, the sheets may be traveling at a relatively high speed, for example, approximately 85 inches per second, while the clock function of the logic and control unit may be relatively slow. Thus if the LCU does not provide timely signals to the applicator valve, adhesive may not flow during the time interval required to provide a continuous stripe of adhesive to the full length of a sheet passing beneath the applicator. Also, an electrically energized system of that type may occasionally respond to stray "noise" signals and turn the applicator valve on or off.
Another difficulty with the adhesive dispensing system disclosed in U.S. Pat. No. 4,406,247 is that the adhesive in the applicator may bleed past fluid seals in the applicator and contaminate the solenoid used for operating the applicator. Such can disable the applicator or cause it to function improperly.
A still further disadvantage of the prior system described in such patent is that it may operate with infrared sensors for detecting the movement of sheets toward the adhesive applicator. Such sensors tend to get dirty, and a dirty sensor can cause a shift in the response time of the system. Also, optical sensors may respond to stray light from a source other than the associated emitter, and they may fail to detect transparencies.